This is from the Humble Demo version of the game.
It seems if you die near the edge of the water, when you re-open the game there's a chance you'll have flown into immovable territory.
So to recreate:
1. Make slime angry.
2. Step near edge.
3. Let angry slime kill you
4. Reload. Be stuck in water.
Managed to eventually recover by adding some Bridge tiles and letting the slime jump at me some more (each death shifted me to the right slightly) but this is not ideal :)